The region around the Sun where the solar wind dominates what goes on in space.

Helium

The second element in the periodic table. Very light compared to normal atmospheric gases (nitrogen and oxygen) and so is used in balloons which then rise in the atmosphere. Liquid helium is very cold and is often used to cool scientific instruments.

Its existence was first seen in spectra of sunlight by Lockyer and others. Here is an image of the Sun from Skylab showing Helium emission.

There is in fact plenty of helium in the Earth's atmosphere (see the brain teaser below) but it is not economic to collect it from there. Instead the helium we use for balloons comes from natural gas wells where it can form up to 7% of the extracted gas.

Brain teaser

Helium is known to form about 5 ppm (parts per million) of the Earth's atmosphere - so how much helium is there all together in our atmosphere?
Standard atmospheric pressure = 105 N/m2 so let's assume that is equivalent to having a mass of air in the atmosphere of about 104 kg/m2 (because 1 kg has a weight of 10 newtons)

Now the radius of the Earth is 6378 km, so the surface area = 5.1 x 1014 m2.
As a result, the total mass of the atmosphere is approximately 5.1 x 1018 kg.

Assuming helium is only 5 ppm of the atmosphere, would give 2.5 x 1013 kg of helium.

On the other hand, although helium, in terms of numbers of atoms, contributes 5 ppm, let's assume the atmosphere is mostly nitrogen. Since a helium atom contributes less than a nitrogen atom to the mass of the atmosphere we just worked out, the mass of helium in the atmosphere we calculated should be reduced by a ratio of 14/4 (the ratio of the atomic masses of nitrogen and helium). A better estimate for the mass of helium in the Earth’s atmosphere is therefore 7.3 x 1012 kg or 7 billion tonnes. Even so, that could fill a good few balloons!

Herschel, Sir William (1738-1822)

William Herschel was born in Hanover Germany and moved to England in 1757. Although he is now remembered as a famous astronomer, astronomy wasn’t originally his main occupation. William loved music, which he studied and taught. His interest in astronomy and mathematics only began quite late in life when he was 35 and read Ferguson's ‘Astronomy’, and Smith's ‘Compleat System of Opticks’. At first he rented small telescopes for his studies, but eventually learned how to grind his own mirrors and so made his own telescopes. One was a reflector with a 48" main mirror, which he built with a £4,000 grant from King George III . He also made a 20' long reflecting telescope, with which he made most of his later observations. All of these later telescopes were massive affairs, far larger than any others available at the time. Herschel discovered many deep-space objects, compiled comprehensive star catalogues and, most famously, discovered a new planet (now called Uranus). This discovery earned Herschel international fame and the continued support of George III. He even attempted, unsuccessfully, to name the planet after the king. That act duly earned him the title of "King's astronomer", but he never became the official "Astronomer Royal". Nevertheless, after his discovery of Uranus, he did receive an annual income from the king and so could devote all of his time to astronomy.

Hertz (Hz)

A unit of frequency (abbreviated to Hz) equal to one cycle, or wave, per second. One kilohertz (kHz) = 1000 Hz and one megahertz (MHz) = 1,000,000 Hz. One GHz = 109 Hz ( 1 billion cycles per second). The unit was named after the German physicist Heinrich Hertz.

Hertz, Heinrich (1857-1894)

A German born physicist and experimenter. He was responsible for the first detection and generation of radio waves. However, Hertz thought his results were of no use and so didn't bother to experiment further. He is supposed to have commented that, "it is of no use whatsoever". Ooops, big mistake!

Hess, Victor (1883-1964)

Austrian-born physicist responsible for the discovery of cosmic rays. Hess published many papers and carried out his research in several institutions. He received many accolades including the Nobel prize for physics in 1936.

Hipparchus of Rhodes (190-120 BCE)

Few personal details are known about this famous Greek mathematician, but we do know that he was born in Nicaea in Bithynia (now part of Turkey), where he carried out many of his studies. Coins have been found in the area decorated with images of him studying a globe. Hipparchus's main achievements were the numerous advances he made in trigonometry. He was the first person to divide the circle into 360°, he calculated the length of the year to within 6½ minutes and compiled a comprehensive star catalogue containing over 850 stars. He also calculated a very accurate estimate of the Earth-Moon distance and discovered that the Earth's axis wobbles in space rather like that of a spinning top (an effect called precession). Not a bad list of achievements!

Hooke, Robert (1635-1703)

The life and work of this outstanding seventeenth-century physicist and mathematician have, until recently, been almost totally in the shadow of his contemporary and rival, Sir Isaac Newton. It is now agreed that many of his achievements were comparable to those of Newton. Hooke was born on the Isle of Wight and educated at Oxford before holding a professorship at Gresham college, London. His most famous discovery is the relationship between the force applied to an elastic material and its extension (Hooke’s Law) . However, he was a prolific inventor and produced the first compound microscope and numerous meteorological instruments. He was also a major contributor to the project to rebuild the city of London after the great fire in 1666.

Huygens, Christiaan (1629-1695)

He was a pioneer in the use of astronomical telescopes and was keen on experimenting with super-long telescopes (up to 100 feet) to give very high magnification. Using one of these instruments at the Paris observatory, he was the first to see and explain the true nature of Saturn's rings. It is, therefore, especially appropriate that part of the Cassini space probe to Saturn was named after him. The Huygens probe detached itself from the main Cassini spacecraft on Christmas day 2004 and parachuted down into the atmosphere of Saturn's largest moon, Titan.

Hydrogen Alpha

The name (often abbreviated to H-alpha, or even Hα) given to light emitted by hydrogen atoms at a wavelength of 656.3 nanometres. This wavelength is in the red portion of the visible spectrum and is emitted especially strongly by gas in the solar chromosphere. This particular wavelength is called alpha because it is the first in a series of wavelengths specifically associated with the hydrogen atom.